Separation system

ABSTRACT

A process for separating a product from a multicomponent feedstream to an adsorption apparatus or system is described. The apparatus or system may comprise a moving-bed or a simulated moving-bed adsorption means. The product comprises at least one organic compound, such as an aryl compound with alkyl substitutes. In embodiments the conduits used to supply the feedstream to the apparatus or system are flushed with media of multiple grades. The improvement is more efficient use of the desorbent. In embodiments the process achieves improvements in one or more of efficiency of adsorption separation, capacity of adsorption apparatus systems, and purity of product attainable by adsorption process.

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Application No. 61/319,080, filed Mar. 30, 2010 and U.S. application Ser. No. 12/774,319, filed May 5, 2010, the disclosures of which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The invention relates to a process for separating one or more of the components from two or more multicomponent fluid mixtures, and more particularly to a process for separating organic compounds from such a fluid mixture by means of adsorption apparatus, such as moving-bed or simulated moving-bed adsorption apparatus, and a system comprising such apparatus.

BACKGROUND OF THE INVENTION

Various means are currently available to separate the components of a multicomponent fluid mixture. If the densities of the components differ sufficiently, the effects of gravity over time may be adequate to separate the components. Depending on the quantities of the components involved, a centrifuge may be used to more rapidly separate components with different densities. Alternatively, distillation may be used to separate components with different boiling points.

Some fluid mixtures comprise components which have similar boiling points, and in such cases, separation by distillation may be a difficult and an inefficient means to separate these components. Too many contaminants, e.g., unwanted components, also may evaporate along with (or fail to evaporate from) the desired component(s), or the separation may require high energy expenditures due to the recycling through the distillation process that may be necessary to attain a desired degree of separation or purity.

In view of these and other deficiencies of these aforementioned processes, adsorption often has been preferred as a process for separating the components from a multicomponent fluid mixture to obtain relatively pure products.

The efficiency of an adsorption process may be partially dependent upon the amount of the surface area of the adsorbent solids which is available for contact with a fluid mixture. The surface area available may be more than just the superficial, external surface of the solids. Suitable solids also may have internal spaces. Such internal spaces may comprise pores, channels, or holes in the surface of the solids and may run throughout the solids, much as in sponges. Thus, the fluid contacts not only the superficial surface, but penetrates into the solids. Sieve chambers increase the contact surface between the fluid and the solids in an adsorption process by concentrating them in a confined space. Such structures often are described as molecular sieves, and the volumetric amount of components that may be adsorbed by a molecular sieve is termed the molecular sieve capacity.

In an adsorption process, separation of the fluid components may be accomplished because the absorbent solid material may have a physical attraction for one or more of the components of the mixture in preference to other components of the mixture. Although all of the components of a mixture may be attracted in varying degrees to the material, there is a preference engineered into the process, such that predominantly the desired component(s) may be attracted and remain with the material in preference over all others. Therefore, even if less preferred components of a mixture initially come into contact with a portion of the material, because of the stronger attraction of the material for the desired component(s) of the mixture, the less preferred component(s) may be displaced from the material by the desired, and more strongly preferred, component(s). Although the fluid mixture entering a sieve chamber might be composed of multiple components, the fluid mixture initially leaving the vessel would be composed largely of the components which had been less preferentially adsorbed into the material.

In adsorption processes using adsorbent solids, separation occurs for a period of time, but eventually all the available surface sites on and in the solids are taken up by the desired component(s) or are blocked by concentrations of unwanted components. At that point, little significant additional adsorption of component(s) from the mixture is likely to occur, and the fluid mixture which might be withdrawn from the chamber may be insignificantly changed by further exposure to the solids. The adsorption step of the process is thus ended, and the component(s) which have been adsorbed by the solids can then be removed from the solids, so as to effect separation and permit reuse of the solids.

A suitable adsorption apparatus or system might first permit adsorption of a product comprising the desired component(s) by the solids and later treat the solids to cause them to release the product and permit recovery of this product. Such an adsorption apparatus or system might comprise a “moving-bed” which permits movement of a tray or bed of the solids through a chamber, such that at different locations, the solid is subjected to different steps of an adsorption process, e.g., adsorption, purification, and desorption. These steps will be understood more clearly by the description below. Nevertheless, moving the solids through an adsorption apparatus may be difficult and involve complex machinery to move trays or beds. It also may result in loss of the solids by attrition. To avoid these problems, some adsorption apparatus and systems have been designed to “simulate” moving the tray(s) or bed(s) to the locations, e.g., zones, of different steps of an adsorption process. Simulation of the movement of the tray(s) or bed(s) may be accomplished by use of a system of conduits which permits directing and redirecting the streams of fluids into the chamber at different zones at different times. As these stream changes occur, the solids are employed in different steps in an adsorption process as though the solids were moving through the chamber.

The different zones within an adsorption apparatus or system are defined by the particular step of the adsorption process performed within each zone, e.g., (1) an adsorption step in the adsorption zone; (2) a purification step in the purification zone; (3) a desorption step in the desorption zone. A more detailed explanation of the zones of the adsorption process follows.

Adsorption Zone: when a multicomponent fluid feedstream, such as a feedstream comprising the C8 aromatics orthoxylene (OX), metaxylene (MX), paraxylene (PX), and ethylbenzene (EB), is fed into the adsorption apparatus or system, the portion of the apparatus or system into which the feedstream is being fed is termed an “adsorption zone.” In the adsorption zone, the fluid comes into contact with the adsorbent material, and the desired component(s) are adsorbed by the adsorbent material. As noted above, other components may also be adsorbed, but preferably to a lesser extent. This preferential adsorption may be achieved by the selection of an adsorbent material, e.g., adsorbent solids, which have a preference for adsorbing the desired component(s) from the multicomponent feedstream. Although only the desired component(s) may have been adsorbed by the solids, other less preferentially adsorbed components of the fluid mixture may still remain in void spaces between the solids and possibly, in the pores, channels, or holes within the solids. These unwanted components preferably are removed from the solids before the desired component(s) are recovered from the solids, so that they are not recovered along with the product.

Purification Zone: after adsorption, the next step is to purify the fluid and adsorbent material in the chamber. In this step, the tray(s) or bed(s) may be moved or flow within the conduits may be changed, so that the multicomponent feedstream may no longer be fed into the adsorption zone. Although the tray(s) or bed(s) have not physically moved, the material may now be described as being in a “purification zone” because a fluid stream, e.g., a purification stream, is fed into the adsorbent material to flush the unwanted components from the adsorbent material, e.g., from within and from the interstitial areas between the solids. Thus, a fluid comprising unwanted components, e.g., raffinate, is flushed from the purification zone by substituting a fluid comprising the desired component(s) or other component(s) deemed to be more acceptable for the unwanted components. The unwanted components may be withdrawn in a raffinate stream. Because an objective of the adsorption process may be to separate the product comprising the desired component(s) from other components which may have nearly the same boiling point or density as the desired component(s), purification may displace unwanted components and substitute another fluid which can be more readily separated by other means, e.g., distilled.

Desorption Zone: after the solids have been subjected to the purification stream, the stream in the conduit(s) may again be changed to introduce a desorbent stream into the chamber to release the product. The desorbent stream contains desorbent which is more preferentially adsorbed by the solids than the product comprising the desired component(s). The desorbent chosen will depend in part upon the desired component(s), the adsorbent materials, and the ease with which the desorbent can be separated from the product. Once the desorbent stream has been introduced to the chamber, the product may be withdrawn from the chamber.

Each and every step and zone might be present somewhere in an adsorption apparatus or system if simultaneous operations are conducted. Nevertheless, the steps may be performed successively or staggered over time. Further, in some adsorption processes, the unwanted components may be adsorbed, and the product comprising the desired component(s) allowed to pass through the adsorption apparatus or system. Therefore, the terms raffinate and extract are relative and may depend upon the particular nature of the components being separated, the preference of the solids, and the nature of the apparatus or system. Although in embodiments the present invention will be discussed primarily in terms of apparatus and systems in which the product is adsorbed by the solids, the invention is not limited to such configurations.

An apparatus suitable for accomplishing the adsorption process of this invention is a simulated moving-bed adsorption apparatus. A commercial embodiment of a simulated moving-bed adsorption apparatus is used in the well-known Parex™ Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene (PX) from a less highly pure mixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717.

Typically, such an adsorption apparatus is contained in a vertical chamber packed with adsorbent solids, possibly in trays or beds stacked within the chamber. More than one type of solid also might be used. The chamber also may have the capability to perform each of the above-described steps simultaneously within different locations, e.g., zones, in the chamber. Thus, the composition of the fluid in the chamber may vary between zones although there may be no structures completely separating these zones. This may be achieved by the use of a serially and circularly interconnected matrix of fluid communication conduits including associated valves, pumps, and so forth, which permit streams to be directed and redirected into different zones of the chamber and to change the direction of these streams through the solids within the different zones of the chamber. The different zones within the chamber may have constantly shifting boundaries as the process is performed.

The cyclic advancement of the streams through the solids in a simulated moving-bed adsorption apparatus may be accomplished by utilizing a manifold arrangement to cause the fluid to flow in a counter current manner with respect to the solids. The valves in the manifold may be operated in a sequential manner to effect the shifting of the streams in the same direction as overall fluid flow throughout the adsorbent solids. In this regard see U.S. Pat. No. 3,706,812. Another means for producing a countercurrent flow in the solid adsorbent is a rotating disc valve by which the streams, e.g., feed, extract, desorbent, raffinate, and line flush, are advanced cyclically in the same direction through the adsorbent solids. Both U.S. Pat. Nos. 3,040,777 and 3,422,848 disclose suitable rotary valves. Both suitable manifold arrangements and disc valves are known in the art. More recently, a system has been described using dual rotary valves. See U.S. application Ser. No. 12/604,836.

Normally there are at least four streams (feed, desorbent, extract, and raffinate) employed in the procedure. The location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. In many instances, one zone may contain a larger quantity of adsorbent material than other zones. Moreover, zones other than those discussed above may also be present. For example, in some configurations, a buffer zone between the adsorption zone and the desorption zone may be present and contain a small amount of adsorbent material relative to the zones surrounding it. Further, if a desorbent is used that can easily desorb extract from the adsorbent material, only a small amount of the material need be present in the desorption zone in comparison to the other zones. In addition, the adsorbent need not be located in a single chamber, but may be located in multiple chambers or a series of chambers.

Introducing and withdrawing fluids to the beds may comprise a plurality of fluid communication conduits, and the same fluid communication conduit may be used in a first instance to input a feedstream into the apparatus or system and later to withdraw an extract stream. This can result in reduced product purity due to contamination of the withdrawn product. Fluid communication conduits may contain unwanted components, such as residue remaining in the conduit from earlier additions or withdrawals of streams. This problem may be overcome by employing separate conduits for each stream or by removing such residue from the conduits by flushing them with a medium which would not effect product purity as adversely as would an unwanted component remaining in the fluid communication conduit. A preferred flushing medium has been the product or the desorbent, which might be more readily separated downstream of the chamber than would the residue. See U.S. Pat. No. 4,031,156. Nevertheless, flushing conduits with the product reduces the output of the adsorption process.

A standard Parex™ unit for separating paraxylene (PX) from the other C8 aromatic isomers, metaxylene (MX), orthoxylene (OX), and ethylbenzene (EB), has a single feed to a single rotary valve or parallel rotary valves. The rotary valve directs the feed to a bed line, which (viewed schematically, such as in the attendant drawings described herein) is somewhere between the extract (which may comprise, by way of example, 99.7% PX and desorbent) and the raffinate (PX-depleted xylenes and desorbent) withdrawal points. Since the process is a simulated moving bed process, the bed lines are shared with all of the feed and product streams, and therefore the bed lines must be flushed between the feed injection point and the extract withdrawal point in order to prevent contamination of the product. A standard unit has a primary flush which removes the majority of contaminants and a secondary flush which removes trace impurities just before the extract point.

The standard commercial simulated moving bed has only a single feed inlet, various streams of different compositions are typically blended together and fed to a single point in the Parex process. However, as indicated in U.S. Pat. No. 5,750,820 (see also U.S. Pat. No. 7,396,973), it is better to segregate feeds which are of substantially different composition, such as concentrated paraxylene from a selective toluene disproportionation unit (generally 85-90% paraxylene) and equilibrium xylenes (generally about 23% paraxylene) from a powerformer, isomerization unit or transalkylation unit. This can be done by using the primary line flush as a second feed point for the paraxylene concentrate and using the secondary flush as the sole flushing stream. Having only a single flush does result in a slight compromise in the separation process, but the compromise typically is far outweighed by the benefit of optimizing the feed location of the paraxylene concentrate as far as net purity in the final product.

There is a problem with the above configuration in that the standard Parex unit has the secondary flush located close to the extract withdrawal point in order to minimize contaminants that are withdrawn with the extract. However, when the secondary flush is very close to the extract withdrawal point and concentrated paraxylene (having associated impurities) is being flushed from the bed line, the configuration will be too close to the extract withdrawal point and the highest separation of the feed will not be realized.

This problem was recently recognized and solved by some of the present inventors. The solution is that the feed locations of both the concentrated paraxylene in the primary flush and also the location of the secondary flush be modified to realize the full benefit of the feed configuration in U.S. Pat. No. 5,750,820. By moving the secondary flush further away from the extract, the material flushed from the bed line will be injected at a more efficient location. See U.S. application Ser. No. 12/774,319. The problem and solution are noted in the description of FIG. 1, herein below.

All of these processes are still very energy-intensive due, at least in part, to the intensive use of materials such as the desorbent, which is typically reused after purification downstream of the bed systems described above. It would be very beneficial if all of the systems described could be modified simply so that energy requirements could be reduced. The present inventors have realized that yet further improvements can be achieved by directly using the first flush output as the secondary flush input. This provides, in embodiments, the ability to completely eliminate purification requirements of the prior art, with its attendant distillation apparatus and pumping equipment, simplifying the system, decreasing costs, and improving the results.

SUMMARY OF THE INVENTION

The invention is directed to a process for separating a product from at least one multicomponent feed by simulated countercurrent adsorptive separation, and to the apparatus or system for accomplishing the same, said process comprising at least two flushing steps to improve product purity, wherein the improvement comprises using the first flush output as the second flush input. In preferred embodiments, there is no step of purification, such as by distillation, the primary flush out material before it is used as secondary flush in.

In embodiments, the process comprises feeding at least two different feeds, the feeds characterized by having different concentrations of at least one product, preferably a C8 species selected from one or more isomers of xylene. It will be recognized by one of skill in the art that a continuous simulated countercurrent adsorptive separation system can have many desired end products, such as pharmaceuticals, fragrances, sugars, and the like.

In embodiments, the feed location of both the primary flush and/or also the secondary flush are altered as compared with the prior art to realize the fullest benefit of the present invention.

In embodiments the conduits used to supply the feedstream to the apparatus or system are flushed with media of multiple grades.

In embodiments the process achieves improvements in one or more of efficiency of adsorption separation, capacity of adsorption apparatus systems, decrease in energy requirements (particularly in the aspects of pumping and distillation requirements, and purity of product attainable by adsorption process.

In an embodiment, the process comprises the steps of: (a) introducing a first multicomponent feed, comprising at least one desired product, through at least one fluid communication conduit into a simulated moving-bed adsorption apparatus comprising at least one rotary valve and plural sieve chambers; (b) flushing the at least one conduit in step (a) with at least one initial flushing medium (which preferably comprises the at least one desired product in step (a) in an initial concentration), whereby residue of said first multicomponent feed is flushed from the at least one conduit in step (a) into the apparatus by the at least one initial flushing medium, so as to produce a primary flush out comprising said at least one initial flushing medium and said residue of said first multicomponent feed; (c) after step (b), flushing said at least one fluid communication conduit with a second and preferably final flushing medium, characterized in that said second flushing medium comprises said primary flush out. In preferred embodiments there is no step of distillation of said primary flush out prior to use as said second flushing medium.

In preferred embodiments, the quantity of the initial medium may not be less than that sufficient to flush the feedstream residue from the conduit.

In embodiments, the apparatus comprises plural sieve chambers containing one or more adsorbent material selected from the group consisting of charcoal, ion-exchange resins, silica gel, activated carbon, zeolitic material, and the like, and the quantity of the initial medium may be sufficient to fill the apparatus to the sieve chamber capacity.

In embodiments, the process includes additional steps including one or more of flushing one or more conduits with a sufficient quantity of a final (or third) flushing medium comprising the at least one desired component in a final concentration, such that the final concentration is greater than the initial concentration and greater than the second concentration, and such that initial medium residue from the at least one initial medium is flushed from the conduit into the system by the final medium; withdrawing a raffinate stream from the system; introducing a desorbent stream to the system; withdrawing a combination comprised of the product and the desorbent from the system; and removing the product from the combination.

In yet another embodiment, the initial concentration of the at least one initial medium is continuously increased during the flushing of the at least one conduit until the initial concentration equals the final concentration. Preferably, this may be accomplished by adding the product to the at least one initial medium in gradually increasing amounts and decreasing proportionately flow from the source of the at least one initial medium; the improvement comprises using at least a portion of the first flush output as at least part of said second flushing medium.

As is well-known per se in the commercial Parex™ unit, moving the locations of liquid input and output is accomplished by a fluid directing device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time, the rotary valve advances one index and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to a new valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time is uniform for each valve step in a valve cycle, and is generally from about 60 to about 90 seconds (although it can be longer or shorter). A typical process contains 24 adsorbent sub-beds, 24 distributors located between the 24 adsorbent sub-beds, two liquid input lines, two liquid output lines, and associated flush lines. In an embodiment of the present invention, an improvement is provided whereby the rotary valve is replumbed so that the input of the secondary flush is a least one and preferably two or more valve steps downstream of where it is, heretofore, ordinarily inputted. This is more fully illustrated by the description of FIG. 1 below. This also means that the secondary flush is added closer in sequence to the input of the primary flush, such as within three cycle steps.

It is an object of the invention to decrease pumping and distillation requirements in the purification of material utilizing a simulated moving bed adsorptive separation system.

It is another object of the invention, in one or more embodiments, to increase the efficiency of adsorption apparatus or systems, whereby contaminants, such as feedstream residue, may be removed from fluid communication conduits by flushing them from the conduits into the apparatus or system with flushing media containing concentrations of the desired component(s) of the product which are higher than that of the feedstream. It is an advantage of such embodiments that if the product is extracted through the same conduits that carried the feedstream, such as in a simulated moving-bed adsorption apparatus, extract will not be contaminated, or will have lower contamination, with feedstream residue.

It is an additional object of this invention to increase the capacity of an adsorption apparatus or system. It is an advantage of this process that excess capacity of the apparatus or system may be more fully utilized by purifying the solids with flushing media and flushing conduits with media containing the desired component(s). It is a feature of such embodiments that fluid communication conduits may be flushed with media containing concentrations of a desired component or components higher than that of the feedstream, which may be drawn from a source other than the apparatus.

It is yet another object of this invention, in embodiments, to increase the purity of the product obtained from an adsorption apparatus or system. It is a feature of embodiments of this process that contaminants may be removed from conduits and from pores, channels, and holes in adsorbent solids, and conduits may be charged with the product. It is an advantage such embodiments that the product may be recycled through the apparatus or system, and excess apparatus or system capacity may be used to further separate other unwanted components of the feedstream remaining in the product.

It is yet still further an object of this invention to eliminate or reduce the circulation requirements of the desorbent, including downstream distillation and separation of the adsorbent from the various solutes contained therein, the solutes including the desired component (such as, in the case of xylenes, one particular isomer, which is generally paraxylene).

These and other objects, features, and advantages will become apparent as reference is made to the following detailed description, preferred embodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views.

FIG. 1 is a schematic illustrating a comparison of various configurations of simulated moving bed adsorptive separation systems.

FIG. 2 is a schematic illustrating one embodiment of a separation system according to the present invention.

DETAILED DESCRIPTION

According to the invention, there is provided a process for separating a product from at least two multicomponent feeds to an adsorption apparatus or system. In an embodiment there is a simulated moving bed adsorptive separation system plumbed so that the first or primary flush output is connected to the secondary flush input directly, whereby the primary flush output, comprising desorbent and having a higher concentration of the desired compound as compared to the flush material prior to being used as primary flush, is then utilized as secondary flush input, resulting, in more preferred embodiments, in the secondary flush output comprising desorbent and having even higher concentration of the desired compound than in the primary flush input.

The apparatus or system may comprise a moving-bed or a simulated moving-bed adsorption means, and in embodiments provides a product comprising at least one organic compound, such as an aryl compound with alkyl substitutes, for instance (and in a preferred embodiment), paraxylene (PX). In embodiments the conduits used to supply the feedstream to the apparatus or system are flushed with media of multiple grades. In embodiments the process achieves improvements in one or more of efficiency of adsorption separation, capacity of adsorption apparatus systems, and purity of product attainable by adsorption process, while allowing for elimination or by-pass of distillation apparatus and/or pumping mechanisms.

In embodiments the feed location of both the concentrated paraxylene in the primary flush and also the location of the secondary flush are located to realize the full benefit of feed locations. In embodiments, by moving the secondary flush further away from the extract, the material flushed from the bed line will be injected into a more advantageous point in the profile. This allows for additional capacity or decreased use of energy associated with a decrease of desorbent recirculation will be realized.

As described more fully in U.S. application Ser. No. 12/774,319, a system employing a simulated countercurrent flow process such as described in U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717, is shown in FIG. 1, along with several modifications. The diagram in FIG. 1 will be understood by those of skill in the art to depict a simulated moving bed process. Desorbent is introduced through conduit 100, flush leaves the apparatus through flush out conduit 101, extract (containing the desired product) leaves the apparatus via conduit 102, raffinate leaves the system through conduit 110, the secondary flush is added through conduit 103, the primary flush is added through conduit 106, a first multicomponent feed is added to the system through conduit 107 and optionally a second multicomponent feed is added through lines 108 or 109, as explained more fully in the following description.

Not shown in the drawing, but as would be recognized by one of skill in the art in possession of the disclosure of U.S. application Ser. No. 12/774,319, is one or more distillation towers and attendant pumps and conduits, which are utilized to purify the flush leaving the above-described apparatus via conduit 101. It has been recognized by the present inventors that such downstream operations can be minimized or entirely omitted by rerouting (such as by replumbing or retrofitting) the primary flush 101 to be used as secondary flush 103. The present inventors have realized that the primary flush 101 has the necessary characteristics of a useful secondary flush, thus avoiding at least part of the circulation of desorbent, with attendant savings in energy, equipment, while at the same time, in embodiments, providing an improved product, e.g., purified PX. An embodiment of the invention will be down further below, with reference to FIG. 2.

Continuing with the description of FIG. 1, the adsorbent 112 moves upward through the sieve chamber vessel 120 containing plural bedlines A₁ through A_(n+j). The hydrocarbon liquid feed 111 flows countercurrent to the circulating adsorbent. In operation, the adsorbent does not flow, but the various feed and product streams cycle through the bed lines, represented by lines A₁ through A_(n+j), at a rate that is different than the circulating hydrocarbon. This simulates the movement of the bed lines A₁ through A_(n+j). Theoretically there may be any number of bed lines, thus n>2 and n+j is the maximum number of bedlines, however from a practical standpoint the number of bed lines is limited by design considerations and other factors. A further discussion may be found in the prior art too numerous to mention, but by way of example the patents discussed in the background above and references cited therein. What is important is the relative positions of the bedlines caused by the stepping of the rotary valve, as would be understood by one of skill in the art (such as that n and j are positive integers and that in typical commercial embodiments the total number of bedlines is 24, and thus n+j typically will be 24). Certain bedlines, i.e., bedlines between A₂ and A_(n), bedlines A_(n+3), A_(n+5), A_(n+6), and A_(n+10) through A_(n+j−1) are not depicted in FIG. 1, for convenience of view.

In a conventional unit, the sieve preferentially starts adsorbing the paraxylene molecules from feed 107 in bedline A_(n+9) and flows upward. In embodiments, the feed is selected from the group consisting of equilibrium xylenes (such as from a powerformer, isomerization unit or transalkylation unit), which is about 21-24 wt % PX, concentrated PX from a selective toluene disproportionation unit (STDP unit), which is about 85-90 wt % PX, and admixtures thereof.

The paraxylene is desorbed from the sieves in the bedlines by desorbent stream 100, the main component of which also is strongly adsorbed on the sieve(s) in bedlines A₁ through A_(n+j), but has a different boiling point and is easily separated from the desired product(s) downstream of the apparatus. In embodiments, the desorbent is paradiethylbenzene (PDEB), toluene, or a mixture thereof, or some other strongly adsorbed compound.

The extract 102, which in the embodiment described is a mixture of the purified paraxylene and the desorbent, is withdrawn at a point between the feed 107 and the desorbent 100. The raffinate 110 consists of the paraxylene-depleted xylenes and desorbent.

Because this is a simulated moving bed process, the various feeds and products must share the lines between the bedlines (sieve beds) and rotary valve (not shown). To prevent loss of paraxylene molecules in the hydrocarbon recycle 111 and subsequently the raffinate 110, the bed lines between the extract out 102 and desorbent in 100 are flushed out, with flush out leaving via conduit 101. The flush out can either be sent to the extract tower for recovery or recycled and used for primary flush in 106.

In addition (and more importantly), since feed 113 is routed through the transfer lines (not shown) between the rotary valve (also not shown) and the sieve chambers A₁ through A_(n+j) before extract 102 the transfer line should be thoroughly flushed to avoid contamination of the product extract 102. Either finished paraxylene product or desorbent, e.g., PDEB or toluene, are routed through a primary flush 114 and a secondary flush 104 (or alternatively 105, as discussed in more detail herein). The secondary flushing step through line 104 is just before the extract withdrawal location 102 in order to flush any trace amounts of contaminants that may have leaked from the sieve chamber(s) back into the bed lines.

As taught in U.S. Pat. No. 5,750,820, it is an improvement to segregate concentrated paraxylene, such as may be obtained downstream from 102 by distillation and then recycle, and use it as primary flush 114 in place of location 106. This step is beneficial because it routes the concentrated paraxylene into a more optimum place in the composition profile. In addition, while the concentrated paraxylene 109 is not as pure as desorbent 100 or product paraxylene (extract 102 is a combination of product para-xylene and desorbent, which can be separated downstream such as by distillation), it does reduce the amount of contaminants in the bed lines A₁ through A_(n+j) and facilitate the secondary flushing step.

However, flushing a bed line (i.e., A₁ through A_(n+j)) full of concentrated PX (85-90 wt %) right next to the extract creates a problem which was unanticipated and not even recognized by the inventors of the aforementioned improvement. Without wishing to be bound by theory, the problem is that following the teaching of U.S. Pat. No. 5,750,820 the flush, in operation, may have about 10 vol % impurities whereas what is needed is closer to zero, such as 0.5 vol % or less impurities.

In previously mentioned U.S. application Ser. No. 12/774,319, the inventors of that case, having discovered the aforementioned problem, proposed that in order to realize the full benefit of the movement of the input of the concentrated stream in line 108 from 107 to 106 via line 109, the feed location of the secondary flush 103 must be moved to an improved place in the composition profile, e.g., further from 104, and closer to 113. One embodiment of such is depicted in FIG. 1 as line 105.

Again, it should be emphasized, as would be known by one of skill in the art, that these positions are relative and that, although the actual positions change by virtue of the movement of the rotary valve (not shown), the relative positions of the lines remains the same. Thus, it will be understood by one of ordinary skill in the art that FIG. 1 depicts a simplified simulated moving-bed apparatus with a rotary valve, wherein countercurrent “movement” of the solids in bed lines A₁ through A_(n+j) relative to the fluid streams is simulated by the use of the rotary valve, which is not shown in the figure. As the valve rotates, the zones previously discussed move through the column in a stepwise sequence due to the change in the stream flows through the valve. In embodiments, a preferred rotary valve for performing this invention is described in U.S. Pat. No. 3,205,166. In this arrangement, each fluid communication conduit connected to the chamber may serve a different function with each step rotation of the rotary valve.

Likewise, with respect to FIG. 2, which is a schematic illustrating an embodiment of the present invention, it will be understood by one of ordinary skill in the art in possession of the present disclosure that FIG. 2 also depicts a simplified simulated moving-bed apparatus with a rotary valve (not shown for convenience of view), wherein countercurrent “movement” of the solids in bed lines B₁ through B_(n+j) relative to the fluid streams is simulated by the use of the rotary valve, which is not shown in the figure. As the valve rotates, the zones previously discussed move through the column in a stepwise sequence due to the change in the stream flows through the valve. As in FIG. 1, in embodiments, a preferred rotary valve for performing this embodiment of the present invention as shown in FIG. 2 is described in U.S. Pat. No. 3,205,166. In this arrangement, each fluid communication conduit connected to the chamber may serve a different function with each step rotation of the rotary valve.

Desorbent is introduced through conduit 200, flush leaves the apparatus through conduit 201, extract (containing the desired product) leaves the apparatus via conduit 202, and raffinate leaves the system through conduit 210. However, unlike in FIG. 1, conduit 201 can be replumbed so as to connect to the conduits 204 and/or 205, analogous to conduit 103 in FIG. 1, to provide secondary flush.

The primary flush is added through conduit 206, a first multicomponent feed is added to the system through conduit 207 and optionally a second multicomponent feed is added through lines 208 or 209, as explained more fully in the following description.

The adsorbent 212 moves upward through the sieve chamber vessel 220 containing plural bedlines B₁ through B_(n+j). The hydrocarbon liquid feed 211 flows (schematically) countercurrent to the adsorbent. In operation, the adsorbent does not flow, but the various feed and product streams cycle through the bed lines, represented by lines B₁ through B_(n+j), at a rate that is different than the circulating hydrocarbon. This simulates the movement of the bed lines B₁ through B_(n+j). Theoretically there may be any number of bed lines, thus n>2 and n+j is the maximum number of bedlines, however from a practical standpoint the number of bed lines is limited by design considerations and other factors. As mentioned above with respect to FIG. 1, a further discussion of these details may be found in the prior art too numerous to mention, but by way of example the patents discussed in the background above and references cited therein. Again, what is important is the relative positions of the bedlines caused by the stepping of the rotary valve, as would be understood by one of skill in the art (such as that n and j are positive integers and that in typical commercial embodiments the total number of bedlines, n+j, is 24). Certain bedlines, i.e., bedlines between B₂ and B_(n), bedlines B_(n+3), B_(n+5), B_(n+6), and B_(n+10) through B_(n+j−1) are not depicted in FIG. 2, for convenience of view.

As in a conventional unit, the sieve preferentially starts adsorbing the paraxylene molecules from feed 207 (bedline B_(n+9)) and flows upward. In embodiments, the feed is selected from the group consisting of equilibrium xylenes (such as from a powerformer, isomerization unit or transalkylation unit), which is about 21-24 wt % PX, concentrated PX from a selective toluene disproportionation unit (STDP unit), which is about 85-90 wt % PX, and admixtures thereof.

The paraxylene is desorbed from the sieves in the bedlines by desorbent stream 200, the main component of which also is strongly adsorbed on the sieve(s) in bedlines B₁ through B_(n+j), but has a different boiling point and is easily separated from the desired product(s) downstream of the apparatus. In embodiments, the desorbent is paradiethylbenzene (PDEB), toluene, or a mixture thereof, or some other strongly adsorbed compound.

The extract 202, which in the embodiment described is a mixture of the purified paraxylene and the desorbent, is withdrawn at a point between the feed 207 and the desorbent 200. The raffinate exits conduit 210, which consist of the paraxylene-depleted (less strongly adsorbed) xylenes and desorbent.

Because this is a simulated moving bed process, the various feeds and products must share the lines between the bedlines (sieve beds) and rotary valve (not shown). To prevent loss of paraxylene molecules in the hydrocarbon recycle 211 and subsequently the raffinate 210, the bed lines between the extract out 202 and desorbent in 200 are flushed out through conduit 201. Appropriately plumbed, which is within the skill of the ordinary artisan in possession of the present disclosure, the flush out can be either be sent to the extract tower for recovery or recycled, used for primary flush in 106, or, as in the embodiment of the present invention, used as secondary flush in through conduit 204 and/or conduit 205. All options may be provided the operator, allowing maximum flexibility.

In addition, since feed 213 is routed through the transfer lines between the rotary valve and the sieve chambers B₁ through B_(n+j) (none of which is shown in the figure for convenience of view but per se using conventional plumbing) before extract 202 the transfer line should be thoroughly flushed to avoid contamination of the product extract 202. Either finished paraxylene product or desorbent, e.g., PDEB or toluene, may still be routed through a primary flush 214 and a secondary flush 204, if desired at predetermined times. The secondary flushing step through line 204 is just before the extract withdrawal location 202 in order to flush any trace amounts of contaminants that may have leaked from the sieve chamber(s) back into the bed lines.

The invention has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims.

Trade names used herein are indicated by a ™ symbol or ® symbol, indicating that the names may be protected by certain trademark rights, e.g., they may be registered trademarks in various jurisdictions. All patents and patent applications, test procedures (such as ASTM methods, UL methods, and the like), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted. When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. 

What is claimed is:
 1. In a process for separating a product from at least one multicomponent feed by simulated countercurrent adsorptive separation, comprising at least two flushing steps to increase purity of the final extract, including a step of obtaining a first flush output and providing a second flush input, the improvement comprising using at least a portion of the first flush output as at least a portion of the second flush input.
 2. The process of claim 1, further comprising the steps of: (a) introducing a first multicomponent feed, comprising at least one desired product, through at least one fluid communication conduit into a simulated moving-bed adsorption apparatus comprising at least one rotary valve and plural sieve chambers; (b) flushing the at least one fluid communication conduit in step (a) with at least one initial flushing medium comprising the at least one desired product in step (a) in an initial concentration, whereby residue of said first multicomponent feed is flushed from the at least one fluid communication conduit in step (a) into the apparatus by the at least one initial flushing medium, so as to produce said first flush output comprising said at least one initial flushing medium and said residue of said first multicomponent feed; (c) after step (b), flushing said at least one fluid communication conduit with a second and preferably final flushing medium, characterized in that said second flushing medium comprises said first flush output.
 3. The process of claim 2, wherein the improvement further comprising using said first flush output as second flush input without a step of distillation between steps (a) and (b).
 4. The process of claim 2, wherein said desired component is an organic compound.
 5. The process of claim 4 wherein said organic compound is a C₈ aromatic isomer.
 6. The process of claim 5 wherein said C₈ aromatic isomer is selected from the group consisting of metaxylene, orthoxylene, and paraxylene.
 7. The process of claim 5, wherein said first multicomponent feed has a concentration of paraxylene of greater than thermodynamic equilibrium with respect to metaxylene and orthoxylene, and the product comprises at least 99.7 vol % paraxylene, based on the total amount of xylenes and ethylbenzene present in said product.
 8. The process of claim 1, wherein said first flush output comprises paraxylene in an amount greater than 25 wt %.
 9. In a process for separating a product comprising paraxylene from at least one feed comprising C8 aromatic hydrocarbons including paraxylene, metaxylene, orthoxylene and optionally ethylbenzene by simulated countercurrent adsorptive separation, said process comprising at least two flushing steps to increase the concentration of paraxylene in said product, including a step of obtaining a first flush output and providing a second flush input, the improvement comprising using at least a portion of the first flush output as at least a portion of the second flush input.
 10. The process of claim 9, further comprising the steps of: (a) introducing a first multicomponent feed comprising C8 aromatic hydrocarbons including paraxylene, metaxylene, orthoxylene and optionally ethylbenzene through at least one fluid communication conduit into a simulated moving-bed adsorption apparatus comprising at least one rotary valve and plural sieve chambers; (b) flushing the at least one fluid communication conduit in step (a) with at least one initial flushing medium comprising paraxylene in an initial concentration, whereby residue of said first multicomponent feed is flushed from the at least one fluid communication conduit in step (a) by said at least one initial flushing medium, so as to produce said first flush output comprising said at least one initial flushing medium and said residue of said first multicomponent feed; (c) after step (b), flushing said at least one fluid communication conduit with a second and preferably final flushing medium, characterized in that said second flushing medium comprises said first flush output.
 11. The process of claim 10, wherein said first multicomponent feed has a concentration of paraxylene of greater than thermodynamic equilibrium with respect to metaxylene and orthoxylene, and the product comprises at least 99.7 vol % paraxylene, based on the total amount of xylenes and ethylbenzene present in said product. 